1. Field of the Invention
The present invention relates to an automatic measuring apparatus for measuring the thickness of a workpiece being polished by detecting a change in the distance between a pair of surface plates, and more particularly, it pertains to such an automatic measuring apparatus which is usable with a lapping apparatus.
2. Description of the Related Art
A lapping apparatus is equipped with an first or upper surface plate and a second or lower surface plate which are rotatably supported through arms so that work pieces each held by a carrier is clamped by means of the upper and lower surface plates which are driven to rotate in opposite directions to thereby polish the work pieces.
In general, with such a lapping apparatus, an automatic measuring apparatus is used to automatically determine whether a work piece is ground or polished to a predetermined thickness.
Conventionally, for such a type of automatic measuring apparatus, there has been known one using a magnetic scale. This type of automatic measuring apparatus is constructed such that the magnetic scale has an upwardly directed probe with its top end being placed in contact with a lower surface of a chip which is coupled with the upper surface plate for integral rotation therewith.
Specifically, as the upper surface plate is moved downward in accordance with an increasing amount of polishing of the work piece, the chip is also moved downward together with the upper surface plate, so that the probe is pushed into the magnetic scale. Thus, the magnetic scale detects an amount of displacement of the probe, whereby the distance between the upper and lower surface plates is measured through the displacement amount, determining the thickness of the work piece.
Another type of automatic measuring apparatus has been known which employs an eddy current sensor 102, as illustrated in FIG. 8. In this automatic measuring apparatus, an eddy current sensor 102 mounted on an upper surface plate 3 radiates a magnetic field toward a lower surface plate 2 so that it detects the distance between the upper and lower surface plates 3, 2 and successively outputs an analog voltage signal indicative of the detected distance. An arithmetic operation means 110 converts the analog voltage signal into a predetermined sampling number of corresponding digital voltage signals, and calculates the distance between the upper and lower surface plates 3, 2 based on the voltage value of the greatest number of digital voltage signals among these digital voltage signals, to thereby determine the thickness of the work piece.
More specifically, the eddy current sensor 102 detects carriers 100 and cross grains or marking grooves 20 on an upper surface of the lower surface plate 2, so analog voltage signals sampled by the arithmetic operation means 110 include, in addition to ones indicative of the distance between the upper and lower surface plates 3, 2, those which indicate the distances from the upper surface plate 3 to the carriers 100 and the cross grains 20.
In this case, an object that is the nearest to the lower surface of the upper surface plate 3 is the upper surface of each carrier 100 disposed on the lower surface plate 2, and the second nearest one is the upper surface of the lower surface plate 2 lying between the carriers 100, and the most distant ones are the cross grains or marking grooves 20.
As a result, the voltage indicating the distance of each carrier 100 to the lower surface of the upper surface plate 3 (hereinafter simply referred to as the carrier distance) is of the lowest value, the voltage indicating the distance of the lower surface plate 2 thereto is higher than the voltage indicative of the carrier distance, and the voltage indicating the distance of the cross grains 20 thereto is of the highest value.
Also, since the detection time of the eddy current sensor 102 detecting the upper surface of the lower surface plate 2 is the longest, the number or frequency of generations of the digital voltage signals indicating the lower surface plate 2 is the greatest. Owing to the fact that the detection time with respect of the upper surfaces of the carriers 100 is less than the detection time for the lower surface plate 2, the frequency of the digital voltage signals indicative of the distances from the lower surface of the upper surface plate 3 to the upper surfaces of the carriers 100 is less the frequency of the digital voltage signals indicative of the distances from the upper surface of the lower surface plate 2 to the lower surface of the upper surface plate 3. Similarly, the detection time for the cross grains 20 is the shortest and hence the frequency relating to this is the lowest.
As a consequence, there is obtained a distribution of frequencies as shown in FIG. 9. As clearly seen from this figure, frequencies C for the carrier 100 appear in a low-voltage side or range; frequencies U for the lower surface plate 2 appear in an intermediate-voltage range, and frequencies S for the cross grains 20 appear in a high-voltage side or range which is slightly higher in voltage than the intermediate-voltage range.
Thus, the arithmetic operation means 110 takes out a voltage value having the greatest frequency of the frequency curve U, and performs a reverse calculation based on this voltage value so as to obtain the distance between the lower and upper surface plates 2, 3, and determines the calculated distance as the thickness of the work piece.
The above-mentioned conventional automatic measuring apparatus have, however, involved the following problems.
First, with the automatic measuring apparatus using the magnetic scale, since it is constructed such that it presumes a quantity of downward displacement of the upper surface plate from a predetermined reference position as an amount of polishing of the work piece being polished, wearing of the lower surface plate results in an error in the measured values.
That is, as the lower surface plate is being worn, the quantity of downward displacement of the upper surface plate, which is equal to the sum of the quantity of wear of the work piece and that of the lower surface plate, can not correspond exactly to the thickness of the work piece. Therefore, when the lower surface plate has been worn to a substantial extent, it becomes impossible to measure the thickness of the work piece in a precise manner.
Furthermore, due to the construction that the probe is placed in contact with the chip integrally rotating with the upper surface plate, the contacting end of the probe is worn by rotation of the chip, thus giving rise to a possible error in measurements.
In contrast to this, with the automatic measuring apparatus employing the eddy current sensor 102, since it is constructed such that the distance between the upper and lower surface plates is detected by using a magnetic field projected to the lower surface plate 2 from the eddy current sensor 102 mounted on the upper surface plate 3, there will be no error due to wear on the lower surface plate 2 and/or the probe.
In this type of automatic measuring apparatus, however, the arithmetic calculation means 110 calculates a voltage value of the greatest number of occurrences or the greatest frequency based on the distribution of the number of occurrences or frequencies as shown in FIG. 9, and then converts it into a corresponding value of thickness of the work piece. With such a construction, there may be a large error in the measurements depending upon the property of the work piece.
Specifically, the work piece is fitted into a corresponding bore in the carrier 100 so that it passes under the eddy current sensor 102. In the event that the work piece is formed of a material having a high resistance, the eddy current sensor 102 can not detect the work piece but instead the upper surface of the lower surface plate 2 disposed under the work piece. In this regard, the frequency curve U as shown in FIG. 9 is the sum of the frequency a of the upper surface of the lower surface plate 2 exposed between the plurality of carriers 100 and the frequency b of the upper surface of the lower surface plate 2 disposed under the work piece, and hence the maximum value of the sum a+b is substantially great in comparison with the maximum value of the frequency curve C. Accordingly, it will be no case in which the frequency curves C and U are distinguished or recognized by mistake.
If the work piece is formed of a material having a low resistance, however, the eddy current sensor 102 can detect the work piece and output an analog voltage signal corresponding to the resistance value.
Thus, the frequency curve of the work piece appears in the vicinity of the frequency curve C, and the maximum frequency of the frequency curve U decreases to the value "a", so that there arises a situation that the maximum values of the frequency curves U, C and the frequency curve of the work piece become substantially similar to each other.
In this situation where the maximum values for the plurality of frequency curves become substantially the same value, the voltage value corresponding to the maximum frequency of the frequency curve U is mistakenly calculated as the voltage value corresponding to the maximum frequency of the frequency curve C, resulting in an incorrect measurement of the thickness of the work piece.